Multi-functional electronic devices

ABSTRACT

Multi-functional electronic switching and current control devices comprising a material capable of supporting a space-charge. The devices include a load terminal, a reference terminal and a control terminal in contact with the space-charge material and a space-charge region is present at each of the multiple terminals, where each space-charge region includes an equilibrium distribution of spatially-separated charged species. Application of a control signal to the control terminal permits a perturbation of the equilibrium of charged species in the space-charge region of either or both of the load terminal and reference terminal. The space-charge perturbations will induce quantum interactions between the space-charge regions associated with the load and reference terminals that will contribute to modulation or inducement of gain, current control, or conductivity mechanism. The devices may be used as interconnection devices or signal providing devices in circuits and networks.

RELATED APPLICATION INFORMATION

This application is a continuation-in-part of U.S. application Ser. No.11/438,709 filed on May 22, 2006, and entitled “Multi-FunctionalChalcogenide Electronic Devices Having Gain”, the disclosure of which ishereby incorporated by reference herein; and a continuation-in-part ofU.S. application Ser. No. 11/446,798 filed on Jun. 5, 2006, and entitledMulti-Functional Electronic Devices Having Gain”, the disclosure ofwhich is hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to chalcogenide electronic devices.More particularly, this invention relates to modification, amplificationand multi-functional switching devices. Most particularly, thisinvention pertains to multi-terminal switching and control devices inwhich the application of a control signal at a control terminalregulates the operation of the device to provide new functionality, in anon-silicon computing platform, that surpasses a transistor.

BACKGROUND OF THE INVENTION

Today's electronic devices rely on conventional silicon technology. Withsilicon technology, one can fabricate the electronic components (e.g.transistors, diodes, switches, memory, integrated circuits andprocessors) needed to produce modern computers and consumer electronicproducts. Silicon-based electronics have enjoyed great success in themarket place and have provided a number of conveniences that havegreatly simplified everyday life.

The growth of silicon-based electronics over the past few decades hasbeen propelled by the enormous strides that have been made in theminiaturization of devices during manufacturing. Miniaturization trendshave faithfully followed Moore's Law for many years over manygenerations of silicon technology. As device feature sizes decrease, itbecomes possible to include ever more devices in a given area of asilicon wafer and to achieve improved performance and speed fromcomputers and electronic products.

Since future improvements in computing power and functionality arecurrently predicated on further improvements in silicon technology,there has been much recent discussion about the prognosis for continuedminiaturization of silicon-based electronic devices. A growing consensusis emerging that believes that the computer industry is rapidlyapproaching the performance limits of silicon. The feature size intoday's manufacturing technologies is 0.18 micron and it is expectedthat this can be reduced to about 0.10 micron in the future. Furtherdecreases in feature size, however, are deemed problematic because sizesbelow about 0.10 micron are expected lead to a change in the fundamentalbehavior of silicon. More specifically, as the dimensions of silicondevices decrease to tens of nanometers and below, silicon enters thequantum regime of behavior and no longer functions according to theclassical physics that governs macroscopic objects. In the quantumregime, energy states are quantized rather than continuous and phenomenasuch as tunneling lead to delocalization of electrons across manydevices. Consequences of tunneling include leakage of current aselectrons escape from one device to neighboring devices and a loss ofindependence of devices as the state of one device influences the stateof neighboring devices. In addition to fundamental changes in thebehavior of silicon, further decreases in the dimensions of silicondevices also pose formidable technological challenges. New innovationsin fabrication methods such as photolithography will be needed toachieve smaller feature sizes.

Two other drawbacks of silicon technology have been identified. First,the costs of installing and operating new manufacturing facilities haveincreased exponentially as feature sizes have decreased. At today's 0.18micron feature size, for example, the cost of building a newsemiconductor fabrication facility readily exceeds a billion dollars.This cost will only increase as devices become smaller and moresusceptible to impurities and process contamination. Second, there isgrowing recognition that the functionality of silicon-based computers isinherently limited as certain computations remain largely unamenable tosolution by modern computers. Examples include factoring, high densityparallel computing, pattern recognition and associative memory.Similarly, many tasks that are readily and intuitively performed byhumans and other biological organisms are difficult, cumbersome andoftentimes impossible to implement with conventional computers.

Consideration of the future of computing indicates a need for newcomputers with new functionality to address ever more sophisticatedapplications. New computers that are adaptable and flexible and thatoperate according to reasoning and intelligence are needed. A needexists for computers that are not limited to the rigid, brute forceproblem solving methodology of conventional computers. Instead,computers are needed that can respond to changing situations with anability to discriminate information from multiple sources to providereasoned outputs, even in the face of seemingly conflictingcircumstances. The functionality required to achieve intelligentcomputers and devices extends beyond the current and projectedperformance capabilities of the silicon technology underlyingconventional computers. Consequently, a need exists for a new andrevolutionary computing paradigm that encompasses general purposecomputers and task-specific computing devices as well as theirunderlying electronic components and materials.

SUMMARY OF THE INVENTION

The instant invention provides electronic devices that can be used inthe construction of novel computers and computing devices having greaterfunctionality than today's conventional computers. The instant devicesare not based on silicon, but rather on non-silicon materials capable ofsupporting a space charge, including chalcogenide phase change materialsthat can be reversibly transformed between resistive and conductivestates. The instant devices can be combined with other devices toachieve a fully functional non-silicon based computing platform or canbe readily interfaced or interconnected with silicon wafers andsilicon-based devices to provide a fully functional hybrid computingplatform.

The instant devices are multi-functional electronic switching andcurrent control devices comprising a material capable of supporting aspace-charge. The devices include three or more terminals in electricalcommunication with the space-charge material and a space-charge regionis present at each of the multiple terminals, where each space-chargeregion includes an equilibrium distribution of spatially-separatedcharged species. The charged species may include electrons, holes,positively-charged atoms and/or negatively charged atoms and may bemobile or stationary. Application of a control signal to one of theterminals permits a perturbation of the equilibrium of charged speciesin the space-charge region of either or both of the other terminals.

The space-charge perturbations may cause one or both space-chargeregions to enlarge or contract and may also induce quantum interactions,such as tunneling, between the space-charge regions associated with theadjacent terminals. These interactions may contribute to the modulationor inducement of gain, current control, or conductivity mechanism.

In one embodiment, the space-charge region of the instant deviceincludes electrons or negative carriers on one side of a contact orjunction and is balanced by holes or positive carriers on the other sideof the contact or junction. The application of a signal to a controlterminal of a multi-terminal device operated in the quantum regimeinduces a change in the ratio of the polarity of positive and negativecarriers within the space-charge regions of one or more terminals of thedevice.

In one embodiment, the space charge material is a phase-change materialthat is capable of undergoing a reversible or irreversible change inphase or structural state upon application of energy. Transformationsbetween states may be effected by providing energy to the phase changematerial in an amount meeting or exceeding a threshold energy.Application of at least a threshold amount of energy from an externalsource to a resistive state causes the material to switch to aconductive state. The conductive state persists as long as a minimumamount of external energy is provided to the material. Upon terminationof the external energy, the material may return to a resistive state. Inother embodiments, the space-charge material is an insulating materialor a semiconducting material.

In one embodiment, the instant device includes three or more terminalsin electrical communication with a material capable of supporting aspace-charge region where a time-varying and/or polarity-varying signalapplied to one of the terminals reversibly modulates the current,current density, conductivity, or threshold voltage between two otherterminals. Reversible modulation may occur while preventing thechalcogenide material of the device from latching.

The instant space-charge gain and switching devices can be linked toother devices to form a network. In a network, the instant switchingdevices may be used as interconnect devices to regulate the flow ofelectrical energy and signals between devices or circuit elements in thenetwork.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. I-V characteristics of a chalcogenide material exhibiting aswitching transformation.

FIG. 2. Schematic depiction of a generic three-terminal chalcogenidedevice according to the instant invention.

FIG. 3. Schematic depiction of an embodiment of a three-terminalchalcogenide device according to the instant invention.

FIG. 4. Illustration of the latching mode of operation of athree-terminal chalcogenide device according to the instant invention.

FIG. 5. Illustration of the non-latching mode of operation of athree-terminal chalcogenide device according to the instant invention.

FIG. 6. I-V characteristics of the three-terminal device illustrated inFIG. 3 showing the switching characteristics of the device between theload and reference terminals as a function of the voltage applied to thecontrol terminal.

FIG. 7. Summary of the relationship between the threshold voltage andholding voltage of the device depicted in FIG. 6 as a function of thevoltage applied to the control terminal.

FIG. 8. I-V characteristics of the three-terminal device illustrated inFIG. 3 showing the switching characteristics of the device between thetop (load) and control terminals as a function of the voltage applied tothe control terminal.

FIG. 9. I-V characteristics of a three-terminal device showing the I-Vresponse of the device between the bottom (reference) and controlterminals as a function of the voltage applied to the control terminal.

FIG. 10. I-V characteristics of the three-terminal device illustrated inFIG. 3 showing the I-V response of the device between the bottom(reference) and control terminals as a function of the voltage appliedto the control terminal.

FIG. 11. Gain characteristics of the three-terminal device illustratedin FIG. 3 showing the current of the device between the top and bottomterminals as a function of the current between the control terminal andthe bottom terminal.

FIG. 12. Current-voltage relationship between the top and bottomterminals of a three-terminal device according to the instant inventionas a function of the current passing the control terminal and the bottomterminal.

FIG. 13. Utilization of a three-terminal chalcogenide device as aninterconnection device between two circuit or network elements.

FIG. 14. Utilization of a three-terminal chalcogenide device as aninterconnection device between three circuit or network elements.

DETAILED DESCRIPTION

The instant invention provides electronic current amplification,modulation, conductivity control and switching devices based onmaterials capable of supporting a space-charge region. In oneembodiment, the material capable of supporting a space-charge is achalcogenide material. The switching properties of the chalcogenidematerials are widely known and have been previously exploited in OTS(Ovonic Threshold Switch) devices. The OTS has been described in U.S.Pat. Nos. 5,543,737; 5,694,146; and 5,757,446; the disclosures of whichare hereby incorporated by reference, as well as in many journalarticles including “Reversible Electrical Switching Phenomena inDisordered Structures”, Physical Review Letters, vol. 21, p.1450-1453(1968) by S. R. Ovshinsky; “Amorphous Semiconductors for Switching,Memory, and Imaging Applications”, IEEE Transactions on ElectronDevices, vol. ED-20, p. 91-105 (1973) by S. R. Ovshinsky and H.Fritzsche; the disclosures of which are hereby incorporated byreference.

The electrical switching properties of the chalcogenide switchingmaterials used in the instant devices are schematically illustrated inFIG. 1, which shows the I-V (current-voltage) characteristics of achalcogenide switching material. The illustration of FIG. 1 correspondsto a two-terminal device configuration in which two spacedly disposedelectrodes are in contact with a chalcogenide material and the current Icorresponds to the current passing between the two electrodes. The I-Vcurve of FIG. 1 shows the current passing through the chalcogenidematerial as a function of the voltage applied across the material by theelectrodes. The I-V characteristics of the material are symmetric withrespect to the polarity of the applied voltage. For convenience, weconsider the first quadrant of the I-V plot of FIG. 1 (the portion inwhich current and voltage are both positive) in the brief discussion ofchalcogenide switching behavior that follows. An analogous descriptionthat accounts for polarity applies to the third quadrant of the I-Vplot.

The I-V curve includes a resistive branch and a conductive branch. Thebranches are labeled in FIG. 1. The resistive branch corresponds to thebranch in which the current passing through the material increases onlyslightly upon increasing the voltage applied across the material. Thisbranch exhibits a small slope in the I-V plot and appears as a nearlyhorizontal line in the first and third quadrants of FIG. 1. Theconductive branch corresponds to the branch in which the current passingthrough the material increases significantly upon increasing the voltageapplied across the material. This branch exhibits a large slope in theI-V plot and appears as a nearly vertical line in the first and thirdquadrants of FIG. 1. The slopes of the resistive and conductive branchesshown in FIG. 1 are illustrative and not intended to be limiting, theactual slopes will depend on the chemical composition of thechalcogenide material and factors such as load resistances in theexternal circuitry. Regardless of the actual slopes, the conductivebranch exhibits a larger slope than the resistive branch and signifies amore freely conducting state of the chalcogenide material than theresistive branch. When device conditions are such that the chalcogenidematerial is described by a point on the resistive branch of the I-Vcurve, the chalcogenide material or device may be said to be in aresistive state. When device conditions are such that the chalcogenidematerial is described by a point on the conductive branch of the I-Vcurve, the chalcogenide material or device may be said to be in aconductive state.

The switching properties of the chalcogenide material used in theswitching embodiments of the instant devices can be described byreference to FIG. 1. We consider a two-terminal device configuration andbegin with a device that has no voltage applied across it. When novoltage is applied across the chalcogenide material, the material is ina resistive state and no current flows. This condition corresponds tothe origin of the I-V plot shown in FIG. 1. The chalcogenide remains ina resistive state as the applied voltage is increased, up to a thresholdvoltage (labeled V_(t) in the first quadrant of FIG. 1). Associated withthe threshold voltage is a threshold current (not labeled). The slope ofthe I-V curve for applied voltages between 0 and V_(t) is small inmagnitude and indicates that the chalcogenide material has a highelectrical resistance, a circumstance reflected in the terminology“resistive branch” used to describe this portion of the I-V curve. Thehigh resistance implies low electrical conductivity and as a result, thecurrent flowing through the material increases only weakly as theapplied voltage is increased. Since the current through the material isvery small, the resistive state of the chalcogenide may also be referredto as the OFF state of the material.

When the applied voltage equals or exceeds the threshold voltage V_(t),the chalcogenide material transforms (switches) from the resistivebranch to the conductive branch of the I-V curve. The switching eventoccurs instantaneously and is depicted by the dashed line in FIG. 1.Upon switching, the device voltage decreases significantly and thedevice current becomes much more sensitive to changes in the devicevoltage. Since the current through the material is greatly increased,the conductive state of the chalcogenide may also be referred to as theON state or the dynamic state of the material.

The chalcogenide material remains in the conductive branch as long as aminimum current, labeled I_(h) in FIG. 1, is maintained. We refer toI_(h) as the holding current and the associated voltage V_(h) as theholding voltage of the device. If the device conditions after switchingare changed so that the current becomes less than I_(h), the materialnormally returns to the resistive branch of the I-V plot and requiresre-application of a threshold voltage to resume operation on theconductive branch. If the current is only momentarily (a time less thanthe recovery time of the chalcogenide material) reduced below I_(h), theconductive state of the chalcogenide may be recovered upon restoring thecurrent to or above I_(h). The recovery time of chalcogenide materialshas been discussed in the article “Amorphous Semiconductors forSwitching, Memory, and Imaging Applications” incorporated by referencehereinabove.

When the current of a device in the ON state is reduced below theholding current, the device relaxes along dotted line labeled “QuantumRegime” in FIG. 1 as the filament collapses. Progress along the dottedline is irreversible, the current continuously decreases until thedevice relaxes back to the resistive branch. FIG. 1 indicates arepresentative placement of the holding current on the I-V curve andspecifically illustrates the common situation in which the holdingcurrent is below the current of the device immediately upon switching.The relative magnitudes of the holding current and current achievedimmediately upon switching depend on factors such as the load resistanceof the device and other factors related to the external circuit in whichthe device is placed. Although it is typical for the holding current tobe less than the current obtained immediately upon switching, thecircuitry and resistances can be configured so as to make the twocurrents coincide. Similarly, the relative magnitudes of the holdingcurrent and the threshold current may vary with the deviceconfiguration, chalcogenide composition, and external circuitry.

Analogous switching behavior occurs in the third quadrant of the I-Vplot shown in FIG. 1. Provided one is cognizant of the negative polarityof the I-V curve in the third quadrant, the switching behavior in thethird quadrant is analogous to that described hereinabove for the firstquadrant. For example, applied voltages having a magnitude greater thanthe magnitude of the negative threshold voltage in the third quadrantinduce switching from the resistive branch to the conductive branch. Wefurther note that although the resistive branch, conductive branch andquantum regimes are depicted with linear representations in FIG. 1, inpractice slightly non-linear or curved representations of these portionsof the I-V curve may be observed. Factors such as loads or resistancesin the circuit external to the device and non-uniformities orimperfections in device fabrication may influence the shape of thedifferent portions of the I-V curve. Accordingly, the depiction providedin FIG. 1 is intended to be schematic and a qualitative representationof the behavior of an actual device in practical operation.

The switching effect of the instant devices originates from atransformation of the chalcogenide material from a resistive state to aconductive state upon application of a threshold voltage, V_(th). Whilenot wishing to be bound by theory, a model can be used to describe thephenomenon underlying the switching transformation. According to themodel, application of the threshold voltage causes the formation of aconductive channel or filament within the chalcogenide material. At thethreshold voltage, the electric field experienced by the material issufficiently high to induce a liberation of charge carriers from bondsor lone pair orbitals of the chalcogenide material. In the liberationevent, electrons are removed from atoms to form a highly conductive,plasma-like filament of charge carriers. Rather than being bound toatoms, some electrons become unbound and highly mobile. As a result, aconductive channel or filament forms. The conductive filamentconstitutes a conductive volume within the otherwise resistivechalcogenide material and may be referred to herein as a solid stateplasma. This solid state plasma has a current density that can exceed10⁷ A/cm². No other solid state plasma is capable of providing a currentdensity anywhere close to this magnitude. The conductive filamentextends through the chalcogenide material between the device terminalsand provides a low resistance pathway for electrical current. Portionsof the chalcogenide material outside of the filament may remainresistive. Since electric current traverses the path of leastresistance, the presence of a conductive filament renders thechalcogenide material conductive and establishes a conductive state. Thecreation of a conductive filament is the event that underlies theswitching of the chalcogenide material from a resistive state to aconductive state. Operation of a chalcogenide device so as to achieve aswitching effect between a pair of terminals may be referred to hereinas a switching mode of operation.

The conductive filament is maintained between the device terminals aslong as the device current remains at or above the holding current. Aconductive filament is present for all points along the conductivebranch, but the cross sectional area of the filament may differ fordifferent points along the conductive branch. The cross sectional areaof the filament refers to directions lateral to the direction of currentflow. Depending on operating conditions within the conductive branch,the filament can be narrow or wide. As the applied voltage is increasedalong the conductive branch, the cross section of the filament isenlarged as the applied voltage is increased. The enlarged filamentindicates a greater volume of the chalcogenide material exhibits highconductivity. As a result, the chalcogenide material can support agreater current, as indicated by the conductive branch of the I-V curve,as the applied voltage increases. Variations of the voltage applied to achalcogenide material operating in the conductive branch modify thewidth or thickness of the filament in directions lateral to thedirection of current flow. The net effect of varying the applied voltageof a chalcogenide material operating in the conductive branch is tomodify the volume fractions of the conductive and resistive portions.The current density may also be influenced.

As will be described in further detail hereinbelow, the instantinvention provides devices having three or more terminals that permitnot only operation along and switching between the resistive andconductive branches of the I-V curve, but also operation at currentsbelow the holding current following switching of the device. This modeof operation may be referred to herein as a subthreshold or quantumcontrol mode of operation and corresponds approximately to the regionsof the I-V curve depicted with a dotted line and labeled “quantumregime” in FIG. 1. In this region, it becomes possible to operate thedevice in a current range below the holding current. Upon switching in aconventional chalcogenide-based switching device, the current jumps inan effectively discontinuous fashion from the low current level in thedevice while in its resistive state immediately prior to switching to ahigher current level at or above the holding current. The range ofcurrent associated with the discontinuity has been heretoforeinaccessible and unavailable for practical device operation. Themulti-terminal devices of the instant invention permit operation of adevice in this otherwise inaccessible range of current and, as describedin greater detail hereinbelow, new functionality is achieved as aresult.

The conductive filament of that forms in the normal operation of achalcogenide device arises from the effect of the electric field imposedon electrons contained in lone pair valence orbitals of the chalcogenelement upon application of a sufficient voltage across the material. Ata sufficiently strong electric field strength, electrons in the lonepair valence orbitals are liberated and collectively form a highlyconducting filament having the characteristics of a solid state plasmaas described hereinabove. Once liberated, a vacancy remains in the lonepair orbital and this vacancy can serve as a recombination center ortrap that removes highly mobile electrons from the filament. In order toprevent collapse of the filament, it is necessary to insure that therate of generation of mobile carriers from the lone pair orbitalsexceeds the. rate of recombination to the vacancies present among thelone pair orbitals. One way to achieve this condition is by maintainingoperation at a current at or above the threshold current of the device.Further opportunities for sustaining a conductive filament are presentedby the instant multi-terminal devices. By providing a signal to acontrol terminal, carriers may be injected into the chalcogenidematerial and these carriers may serve to fill or passivate emptyvacancies or traps that would otherwise serve as recombination centers.As described more fully in the discussion and examples presentedhereinbelow, it thus becomes possible to maintain a highly conductiveregion or filament within a chalcogenide material even at currents belowthe threshold current. In addition, the presence of a signal at thecontrol terminal may also influence the structural state of thechalcogenide through, for example, reorientations of polymeric chains oratomic or molecular groups within chalcogenide materials and suchstructural effects may influence the energy required to liberateelectrons or charge carriers from lone pair orbitals. This effect may,for example, facilitate the modification of the characteristics of achalcogenide material in the bulk of the material away from the regionadjacent to the control terminal or other terminal to which a signal isapplied.

Chalcogenide materials of many chemical compositions undergo theforegoing switching effect. Representative chalcogenide materials arethose that include one or more elements from column VI of the periodictable (the chalcogen elements) and optionally one or more chemicalmodifiers from columns III. IV or V. One or more of S, Se, and Te arethe most common chalcogen elements included in the active material ofthe instant devices. The chalcogen elements are characterized bydivalent bonding and the presence of lone pair electrons. The divalentbonding leads to the formation of chain and ring structures uponcombining chalcogen elements to form chalcogenide materials and the lonepair electrons provide a source of electrons for forming a conductingfilament. Trivalent and tetravalent modifiers such as Al, Ga, In, Ge,Sn, Si, P, As and Sb enter the chain and ring structures of chalcogenelements and provide points for branching and crosslinking. Thestructural rigidity of chalcogenide materials depends on the extent ofcrosslinking and leads to a broad classification of chalcogenidematerials, according to their ability to undergo crystallization orother structural rearrangements, into one of two types: thresholdmaterials and memory materials.

Both types of chalcogenide materials display the switching behaviorshown in FIG. 1, but differ in their structural response to filamentformation. Threshold materials generally possess a higher concentrationof modifiers and are more highly crosslinked than memory materials. Theyare accordingly more rigid structurally. Threshold materials areamorphous and show little or no tendency to crystallize because theatomic rearrangements required to nucleate and grow a crystalline phaseare inhibited due to the rigidity of the structure. The structuralrigidity also aids in reducing leakage currents. Threshold materialsremain amorphous upon removing the applied voltage after switching.

Memory materials, on the contrary, are more lightly crosslinked and moreeasily undergo full or partial crystallization. An amorphous memorymaterial undergoes filament formation in the presence of a thresholdvoltage as described in FIG. 1 hereinabove. Once in the conductivebranch, however, the memory material may undergo nucleation and growthof a crystalline phase. The volume fraction of the crystalline phasedepends on the magnitude and time of the current passing through thememory material. The crystalline phase is retained upon removing theapplied voltage after switching. Through appropriate selection of deviceoperating conditions, the amorphous-crystalline transformation ofchalcogenide memory materials becomes reversible over many cycles.Chalcogenide memory materials have been discussed in U.S. Pat. Nos.5,166,758; 5,296,716; 5,534,711; 5,536,947; 5,596,522; and 6.087,674;the disclosures of which are hereby incorporated by reference.

Embodiments of the instant devices improve upon the prior arttwo-terminal devices by providing multi-terminal devices with which itis possible to control the operating conditions required to induceswitching and filament formation in a desired location within achalcogenide material, by providing devices that permit operation atotherwise inaccessible currents below the holding current, and byproviding devices that exhibit gain or amplification of current. Someembodiments of the instant devices include one or more input terminals,one or more output terminals, and one or more control terminals inelectrical communication with a chalcogenide material. In the instantdevices, a suitable control signal at the control terminal influencesthe conductivity, threshold switching voltage or gain factor of thechalcogenide material between a first terminal of the device and asecond terminal of the device. In the absence of a control signal, thechalcogenide material switches from a resistive state to a conductivestate upon application of a threshold voltage, where the magnitude ofthe threshold voltage corresponds to the threshold voltage between thefirst and second terminals in the corresponding two-terminal deviceconfiguration. The presence of a suitable control signal at the controlterminal of the instant multi-terminal devices permits modulation of thethreshold voltage between the first and second terminals to a magnitudedifferent from that obtained in the corresponding two-terminal deviceconfiguration. Also, when a sub-threshold voltage is applied between afirst terminal and a second terminal of the instant devices, applicationof a control signal may vary the conductivity of the chalcogenidematerial between the two terminals by inducing a transformation of thechalcogenide material from a resistive state to a conductive state. As aresult, high conductivity may be achieved between one terminal andanother terminal even when the voltage applied between the two terminalsis below the threshold voltage of the corresponding two terminal device.In these embodiments, the control signal may be an electrical signalsuch as a current or voltage.

In other embodiments of the instant invention, the control signal is anoptical signal. Devices according to these embodiments include a firstterminal and a second terminal in electrical communication with achalcogenide material, wherein an optical control signal provided by anoptical source is used to control the conductivity or threshold voltageof the chalcogenide material between the first and second terminals. Theoptical control signal provides energy to the chalcogenide material andmay be applied to selected portions of the chalcogenide material or tothe whole of the chalcogenide material. Suitable optical sources includeincandescent lights, lasers, diodes, light provided by optical fibers orwaveguides or light provided by optical chalcogenide materials,including those that contain Se. Optical sources operating in continuousmode or pulsed mode are within the scope of the instant invention.

In other embodiments, the instant devices provide for a gain oramplification of current in a chalcogenide device having three or moreterminals. In these embodiments, a voltage is applied between a firstterminal and second terminal of the device and is maintained at a levelbelow the threshold voltage. While maintaining the subthreshold voltage,a control signal (e.g. voltage) is applied to a third terminal of thedevice to produce or maintain a subthreshold current flow between thefirst and second terminals. The control signal further establishes aflow of current to/from one of the first and second terminals from/tothe third terminal. Once a current is established with the thirdterminal, subsequent variation of the signal applied to the thirdterminal may produce gain or amplification of the current between thefirst and second terminals. In one embodiment, the third terminalinjects current carriers that contributes to the current flowing betweenthe first and second terminals. The net effect of this embodiment isachievement of a multiterminal chalcogenide device that hastransistor-like functionality. Operation of a chalcogenide device so asto achieve transistor-like functionality may be referred to herein as again or transistor mode of operation. In one embodiment, the device canbe operated in gain mode between one pair of terminals. In anotherembodiment, the device can be operated in switching mode between anotheror the same pair of terminals. In yet another embodiment, the device canbe operated in gain mode between one pair of terminals and switchingmode between another pair of terminals.

One embodiment of the instant devices is a three terminal device havinga first terminal, a second terminal and a control terminal in electricalcommunication with a chalcogenide material. A schematic depiction ofthis embodiment is presented in FIG. 2. In this embodiment, the controlterminal may be used to modulate the conductivity of the chalcogenidematerial between the first and second terminals or to modulate thethreshold voltage that must be applied between the first and secondterminals to induce a transformation of the chalcogenide material from aresistive state to a conductive state. The first and second terminalsmay also be referred to herein as non-control terminals, input and/oroutput terminals, or load and reference terminals. Terminals may also bereferred to as electrodes and may include terminals containing a singlelayer or chemical composition as well as terminals comprising two ormore layers.

Another embodiment of the instant devices is a three terminal devicehaving a first terminal, a second terminal and a control terminal inelectrical communication with a chalcogenide material. In thisembodiment, the control terminal may be used to create or modulate thegain or amplification of current flow through the chalcogenide materialbetween the control terminal and one of the first and second terminalsor between the first and second terminals. In a preferred embodiment,the structure of the device includes an upper or top terminal, a loweror bottom terminal and an intermediate terminal positioned or spacedlydisposed between the upper and lower terminals, where the intermediatecontact serves as the control terminal. In another preferred embodiment,the control terminal is spatially positioned closer to one of the upperor lower terminals and a signal applied to the control terminal orbetween the control terminal and the closer of the upper or lowerterminals produces a gain or amplification of the current passingbetween another pair of terminals of the device.

Embodiments of the instant invention include devices having a structurein which one terminal is spacedly disposed between two other terminals,where the spacedly disposed terminal is located in closer proximity toone of the two terminals between which it is positioned. Suchembodiments may be referred to herein as asymmetric device structures tosignify an asymmetric positioning of the spacedly disposed terminal. Ina preferred embodiment, the spacedly terminal is a control terminal.Devices in which the spacedly disposed terminal is equally situatedbetween two other terminals may be referred to herein as symmetricdevices.

Other embodiments of the instant invention include those in which thechalcogenide material, although switchable in principle, may not beswitched between a pair of terminals of a multi-terminal device. In apreferred embodiment, the spatial separation between a pair of terminalsis kept sufficiently small to inhibit or prevent switching of thechalcogenide material.

Further embodiments of the instant invention include electronic deviceshaving three or more terminals in electrical communication with amaterial capable of supporting a space-charge. A material capable ofsupporting a space-charge may be referred to herein as a space-chargematerial. In these embodiments, a space-charge region is present at eachterminal of the device, where the space-charge region includes adistribution of charged species. The charged species may includeelectrons, holes, positively-charged atoms or ions, and/ornegatively-charged atoms or ions. The charged species may be mobile orstationary. In a preferred embodiment, the charged species are spatiallyseparated. In the quiescent state of the device, an equilibriumdistribution of charged species is present at each terminal.

In one embodiment, the device includes three terminals. The threeterminals may be referred to as a control terminal, a load terminal anda reference terminal. In this embodiment, when a signal is applied tothe control terminal, the equilibrium distribution of charged speciespresent in the space-charge regions of either one or both of the loadterminal and reference terminal is modified. The relative proportions ofcharged species may be altered, charged species may be eliminated orcreated and/or the spatial extent of the space-charge region may beenlarged or contracted. The control signal permits a modulation of thephysical dimensions (which are preferably in the limit of quantumdimensions) and/or electrical characteristics, including thedistribution of charged species, of the space-charge region at one ormore terminals spatially disposed from the control terminal. In oneembodiment, the control signal injects electrons into or removeselectrons from the space-charge region at the load terminal and/orreference terminal. In another embodiment, the control signal injectsholes into or removes holes from the space-charge region at the loadterminal and/or reference terminal. In still another embodiment, thecontrol signal creates an electric field that modifies the space-chargeregion at the load terminal and/or reference terminal through a fieldeffect without the injection of a charge carrier. The control signal maybe a voltage or current signal and may be DC or time-varying (e.g. AC orpulsed).

The modulation of space-charge region afforded by the instant inventionis a universal principle that we make manifest in any material capableof supporting a space-charge region. Space-charge sustaining materialsinclude, for example, semiconductors having any bandgap and insulators.In one embodiment, the space-charge material is a chalcogenide material.In another embodiment, the space-charge material is a phase changematerial.

While not wishing to be bound by theory, the instant inventor believesthat through control of the space-charge regions at the differentterminals, it become possible to control the electrical characteristicsof the space-charge material and as a result, the electrical propertiesof devices based on a space-charge material. In the case of chalcogenidematerials, the current and voltage characteristics of a switchingtransition between the load and reference terminals may be controlled bymodulating the space-charge region in the vicinity of either or both ofthe load terminal or reference terminal through application of a controlsignal at the control terminal. Upon application of a signal at thecontrol terminal, an electric field may develop at the interface betweenthe terminal and the adjacent space-charge material and that thiselectric field may extend into and influences the characteristics of thespace-charge region at either or both of the load terminal or referenceterminal. The control signal may also inject or remove electrons orholes or otherwise alter the equilibrium distribution of charge carriersin either or both of the space-charge regions. The occupancy of traps,defect states or surface states of the space-charge material in theaffected space-charge region(s), for example, may be influenced by theelectric field and/or injection or removal of charge carriers. Defectsor atoms may be ionized within one or more space-charge regions locatedat the load terminal or reference terminal upon application of a controlsignal. Judicious selection of the duration, intensity, width, andtime-variance of the control signal permits modulation of thespace-charge regions located at one or more terminals of the device.

When neighboring terminals are sufficiently far apart spatially, theirrespective space-charge regions do not overlap and a switchingtransformation can be effected as described hereinabove between theterminals. As the spatial separation between terminals decreases,however, it becomes possible for the space-charge regions of neighboringterminals to perceive or otherwise influence each other. At sufficientlysmall separations, the space-charge regions may even overlap spatially.

When the space-charge regions of neighboring terminals do not interactor overlap by virtue of the spatial position of the terminals and/or themagnitude of the signal applied across the neighboring terminals,application of a control signal at a control terminal may induce suchinteractions or overlapping. As described hereinabove, the applicationof a control signal can modulate the spatial extent and distribution ofcharged species of the space-charge regions of other terminals of thedevice. The control signal may, for example, induce an enlargement ofthe physical size and/or electric field strength of one or morespace-charge regions and thereby create an interaction or overlapbetween adjacent space-charge regions. The interaction may be aclassical coulombic or electrostatic interaction or a quantuminteraction. In one embodiment, the quantum interaction is a tunnelingphenomenon in which a charged species from one space-charge region istransmitted to or delocalizes on a neighboring space-charge region toproduce a redistribution of charged species between or within two ormore space-charge regions.

Different regimes of device operation may be envisioned based on themutual interactions of the space-charge regions emanating from orestablished adjacent to different terminals. In one regime, theterminals are sufficiently separated (or the space-charge regions aresufficiently contracted or weak) to prevent significant interactionsbetween the space-charge regions of neighboring terminals. This regimeis believed to correspond, for example, to the operating regime ofconventional chalcogenide switching devices. A second and unique regimeis one aspect of the instant invention and transforms the device intomulti-functional quantum control devices capable of operating accordingto unique quantum functionality. In the manifestation of this regime, atleast one pair of terminals is sufficiently close together (or at leastone space-charge region is appropriately modified in spatial extent,electronic or quantum characteristics through a control signal) toenable their space-charge regions to interact. The interaction may becoulombic or electrical in nature and may be based on well-knowninteractions between electric or charge fields in general. Suchinteractions may be repulsive or attractive in character. Quantuminteractions, such as tunneling, may occur between space-charge regionsof neighboring terminals in this regime if the space-charge regions aresufficiently close together. The space-charge regions emanating fromneighboring terminals may, for example, extend 60 Å. If the terminalsare spaced 125 Å apart, the space-charge regions do not overlap, butthey are nonetheless close enough together (5 Å between the outerboundaries) that tunneling, exchange or other quantum effects may occur.In a third regime, at least one pair of terminals is sufficiently closetogether (or at least one space-charge region is sufficiently spatiallyextended or strong) to enable their space-charge regions to spatiallyoverlap. It is to be noted that for a given (or fixed) spatialseparation of terminals, the magnitude of signals (e.g. voltages orcurrents) applied to the different terminals may influence the regime inwhich the device operates so that for a particular device structure,operation in one or more regimes may be possible depending on the natureor strength of the applied signals. Furthermore, as describedhereinabove, a control signal applied away from a particular terminalmay be used to manipulate or modify the characteristics of thespace-charge region at that terminal.

In customary terms, it is believed that chalcogenide switching devicesmay reside either in a resistive state or a conductive state and thatthe intervening electrical or physical states that bridge the conductiveand resistive states are transitory and fleeting and incapable of beingstabilized or otherwise harnessed for beneficial effect. The instantinventor believes that this view is most closely associated with thefirst of the three regimes described above, the regime in which nosignificant interaction is present between the space-charge regions ofneighboring terminals during device operation. In this regime, thepresence of a space-charge region at one terminal does not materiallyaffect the characteristics of the space-charge region at anotherterminal. As described in further detail in the EXAMPLES presentedhereinbelow, by controlling the relative spatial positions of theterminals, the instant inventor has enabled interactions betweenspace-charge regions of neighboring terminals during device operationand new operational features, including gain and reversible modulationof currents at levels below the holding current and/or threshold currentwhen the space-charge material is a chalcogenide material, have beenachieved for the first time as a result. By controlling the relativeproximities of space-charge regions and inducing interactions betweenthe space-charge regions, it becomes possible to alter the conventionalprocesses associated with creating a filament and controlling theconductivity of a chalcogenide material to achieve new functionality.Rich new physics is achieved. Comparable effects based on the intrinsicproperties of other families of space-charge materials also occur in theinstant devices.

The quantum regime is achieved by: (1) operating within quantum levelnano-dimensions; (2) wavefunction engineering within the quantumdimensions; (3) altering, modulating, and controlling the quantum areand regime through the base material that supports a space-chargeregion; (4) controlling the polarities of the pulses or analogelectronic regime; (5) using alloying and other means to change the basematerials and materials employed as control electrodes; and/or (6)utilizing as many as four space-charge area, not only to control, butalso to make in one nanostructure device an operating circuit where eachwavefunction area interacts with others to have various kinds of newcircuits having logic and other functionality.

Through manipulations of the space-charge regions of multi-terminaldevices, the instant invention permits mutually interactions between thespace charge regions associated with any combination of two or moreterminals of the device. The effects described hereinabove, includingspace-charge overlap, quantum interactions and wavefunctioninteractions, can be established and controlled between any combinationof terminals through the design of the device. Pairwise interactions,for example, between the space-charge regions associated with any pairof terminals in a multi-terminal device may be induced, sustained andcontrolled to realize the beneficial effects of the quantum regimedescribed herein. Mutual interactions between space-charge regions canserve as the basis for logic functionality (including binary andnon-binary logic operations) and a quantum computing capability. Singlechips can be fabricated that combine logic, processing, and memory in acompact, all thin film package using cost-effective fabricationtechniques.

In some embodiments of the instant invention, the electricalcommunication between a terminal and the space-charge (e.g.chalcogenide) material of the instant devices is direct, whereby anelectric current propagates from the terminal to the chalcogenidematerial. A terminal that influences the chalcogenide material directlymay be referred to herein as a direct terminal or direct contact. In oneembodiment, a direct terminal may inject charge carriers (e.g.electrons, holes). In other embodiments of the instant invention, theelectrical communication between a terminal and the chalcogenidematerial of the instant devices is indirect, whereby an electricaleffect at the terminal (such as a potential, charge accumulation orelectric field) influences the chalcogenide material without passage ofan electrical current. A terminal that influences the chalcogenidematerial indirectly may be referred to herein as a field effect terminalor a field effect contact. In other embodiments, a terminal mayinfluence the chalcogenide material through a magnetic orelectromagnetic interaction.

In one embodiment, a terminal includes a conductive material in contactwith a barrier material where the barrier material is in contact withthe chalcogenide material of the device. In another embodiment, aterminal includes a conductive material and a barrier material whereelectrical communication between the conductive material andchalcogenide material occurs through the barrier material. In stillanother embodiment, a terminal includes a chalcogenide material disposedbetween two conductive materials where one of the conductive materialsis in electrical communication with the working chalcogenide material ofthe instant devices. In this embodiment, the terminal may be atwo-terminal chalcogenide device, such as an Ovonic Threshold Switch,where the conductivity of the terminal is controlled by the resistivityof the chalcogenide material included in the terminal. A memory typechalcogenide material may also be used. In these embodiments, theterminal can be resistive or conductive and thereby control access of acontrol signal or input signals to the working chalcogenide of theinstant devices. In yet another embodiment, a terminal may be a fieldeffect electrode that includes a thin dielectric layer interposedbetween a conductive material and the working chalcogenide. Principle ofoperation of such an electrode is similar to that of a gate electrode ina MOSFET. The dielectric layer inhibits the flow of current from theconductive material to the working chalcogenide, but is sufficientlythin to allow electric fields present in the conductive material toinfluence the chalcogenide material.

Analogous embodiments having more than three terminals are also withinthe scope of the instant invention. In these embodiments, devices havinga plurality of input, output and/or control terminals are within thescope of the instant invention. Preferred embodiments of the instantinvention are those in which different terminals are electricallyisolated from one another in the sense that electrical communication orsignal transmission between any pair of terminals occurs through thechalcogenide material. Electrical communication and signal transmissioninclude the communication or transmission of electrical effects such ascharges, currents or voltages. Electrical isolation may occur, forexample, by separating electrodes with an insulating material or byotherwise spacedly disposing the electrodes. The instant inventionincludes embodiments in which three or more terminals in electricalcommunication with a chalcogenide material are arranged in a planarconfiguration as well as embodiments in which the terminals are arrangedin a non-planar configuration.

In U.S. Pat. Nos. 6,967,344 and 6,969,867, the disclosures of which areincorporated by reference herein, the instant inventors describedthree-terminal chalcogenide devices having a control terminal capable ofinjecting current into a chalcogenide pore region interposed between anupper and lower contact. Embodiments demonstrating injection throughdirect and indirect (field effect) means were disclosed. The patentsfurther demonstrated that application of a suitable electrical signal atthe control terminal permitted modulation of the threshold voltage of achalcogenide switching material. Notably, it was shown that the presenceof a voltage or other electrical signal at the control terminal couldreduce the voltage required to trigger a switching event between theupper and lower contacts.

Similar injection and modulation effects are among the features of theinstant devices, as will be described in the Examples presentedhereinbelow. The instant devices further provide for a gain oramplification of current as well as a reversibility in the ability tomodulate the threshold voltage of a chalcogenide switching material. Aswill be described in some of the Examples hereinbelow, the instantdevices include a structural arrangement or positioning of the controlterminal relative to the top or bottom terminal that provides for asubthreshold mode of operation that inhibits a chalcogenide switchingdevice from latching into its ON state from its OFF state when a signalis applied at the control terminal. By avoiding latching, the theeffects of applying a signal at the control terminal becomes reversible.The threshold voltage, for example, required to effect a switching eventbetween the top and bottom contacts can be reversibly modulated simplyby varying the magnitude of the signal applied to the control terminal.Since the device does not latch in the subthreshold mode of operation,it is not necessary to relax the device from its ON state back to itsOFF state in order to modulate the threshold voltage.

Embodiments of the instant invention include devices having structuresin which the control terminal is spacedly disposed between the upper andlower terminals and is located in closer spatial proximity to the lowerterminal or contact than to the upper terminal or contact. Otherembodiments of the instant invention include devices having structuresin which the control terminal is located in closer spatial proximity tothe upper terminal or contact than to the lower terminal or contact. Thedevice structures of these embodiments may be referred to herein asasymmetric devices structures to signify the fact that the controlterminal is not equally spaced between the upper and lower contacts. Theasymmetric device structure may facilitate subthreshold or non-latchingoperation of a chalcogenide device and leads to the beneficial gain andreversibility effects described herein.

Another embodiment of the instant invention includes chalcogenideelectronic devices having three or more terminals where application of atime-varying signal to one of the terminals reversibly modulates thecurrent, current density, conductivity, and/or threshold voltage of thechalcogenide material between a pair of terminals of the device.

Another embodiment includes chalcogenide electronic devices having threeor more terminals where the device can simultaneously or sequentiallytransmit currents between two or more pairs of terminals withoutswitching or transforming from a resistive state to a conductive state.

Another embodiment includes chalcogenide electronic devices having threeor more terminals where it is possible to switch the chalcogenidematerial between one pair of terminals from a resistive state to aconductive state, but where the chalcogenide material cannot be switchedfrom a resistive state to a conductive state between a different pair ofterminals.

Another embodiment includes chalcogenide electronic devices having threeor more terminals where the terminals are arranged in an asymmetricconfiguration in which one terminal is spacedly disposed between twoother terminals and is located in closer spatial proximity to one of thetwo surrounding terminals than to the other of the two surroundingterminals.

EXAMPLE 1

An example of a device structure according to the instant invention isshown in FIG. 3. FIG. 3 shows a cross-sectional view of a three terminaldevice structure. A plurality of these devices was formed on a siliconwafer. The devices and layers on the wafer were formed usingconventional sputter deposition, etching, and lithography techniques.The structure includes a silicon wafer substrate 10, a silicon oxidelayer 20, a bottom terminal that includes a conductive layer 40 formedfrom Mo and a carbon layer 50, a lower Si₃N₄ insulating region 60, acontrol terminal that includes a carbon layer 65 and a Mo layer 70, anupper Si₃N₄ insulating layer 75, a chalcogenide material 80 contained inthe pore region of the device, a top terminal that includes a carbonlayer 85 and a conductive layer 90 that includes Mo, and Al layers 95.In this example, the chalcogenide material 80 is an Se-Te chalcogenidealloy having an approximate composition Si₅Ge₁₁As₂₈Te₃₄Se₂₁S. Typicallayer thicknesses are indicated in FIG. 3. The region occupied by thechalcogenide material in device of this example is cylindrical with aheight of approximately 0.1 micron. The region occupied by thechalcogenide material may be referred to herein as a pore, pore regionor the like. The upper, lower, and control terminals are in electricalcommunication with the chalcogenide and correspond to the terminalsindicated in the depiction of FIG. 2. Carbon layer 65 of the controlterminal circumscribes the chalcogenide material 80. The top terminaland bottom terminal may also be referred to as the load and referenceterminals, respectively. In a preferred embodiment, the referenceterminal is at ground. The terminals are separated by insulatingmaterial so that electrical communication between electrodes occursthrough the chalcogenide material.

The device depicted in FIG. 3 may be referred to herein as an asymmetricdevice because of the asymmetry in the position of the control terminal70 relative to the top terminal 90 and bottom terminal 30. Morespecifically, the thickneses of the insulating layers 60 and 65 are suchthat the control terminal 70 is located in closer spatial proximity tobottom terminal 30 than to top terminal 90. The asymmetric placement ofthe control terminal 70 facilitates the operation of the device in asubthreshold mode as well as operation of the device without latching.The control terminal 70 is spacedly disposed between bottom terminal 30and top terminal 90. The separation between the control terminal 70 andbottom terminal 30 is believed to be sufficiently close to permit anoverlap or interaction of space-charge regions emanating from theterminals.

EXAMPLE 2

In this example, a device having a symmetric design was fabricatedaccording to the process described in EXAMPLE 1 hereinabove. In thesymmetric design, the upper insulating layer 75 and lower insulatinglayer 60 both had a thickness of 500 Å, so that the control terminal 70was positioned symmetrically between top terminal 90 and lower terminal30. Other features of the symmetric device are as described inEXAMPLE 1. The separation between control terminal 70, top terminal 90and lower terminal 30 is believed to be large enough to prevent asignificant interaction between the space-charge regions emanating fromthe three terminals.

EXAMPLE 3

In this example, a demonstration of the latching and non-latching modesof operation of the device described in EXAMPLE 1 is provided. Anillustration of operation in latching mode is presented in FIG. 4, whichshows the time variation of the voltage between the top and bottomterminals of the device before and after application of a voltage signalbetween the control terminal and bottom terminal. FIG. 4 includes twodata curves: one, depicted in diamond symbols, that shows the voltagebetween the top and bottom terminals as a function of time and another,depicted in square symbols, that shows the voltage between the controland bottom terminals as a function of time. At the outset of theexperiment, a signal of approximately 2V was applied between the top andbottom terminals and no signal was applied between the control terminaland the bottom terminal. The signal applied between the top and bottomterminals was an insufficient voltage to cause a switching event tooccur. While the signal was maintained between the top and bottomterminals, a control signal of about 4.5V was applied between thecontrol terminal and bottom terminal. Upon application of the controlsignal, the voltage between the top and bottom terminals was observed todrop to a level of slightly above 1V. The voltage drop signifies theinducement of a switching event between the top and bottom terminals bythe control signal in which the ON state of the chalcogenide material isestablished. Upon removal of the control signal, the data shown in FIG.4 indicate that the device remained in its ON state as the voltagebetween the top and bottom terminals remained at approximately the samevalue as existed while the control signal was applied. The removal ofthe control signal from the control terminal did not cause the device torelax from its ON state. The device remained in its ON state and thecurrent passing between the top and bottom terminals remained at orabove the holding current. This behavior demonstrates the latchingcapability of the instant device.

Results of a second experiment are shown in FIG. 5, which illustratesthe non-latching mode of operation of the instant device. In thisexperiment, the initial voltage between the top and bottom terminals wasset to a value of about 1.5V before the control signal of about 4.5V wasapplied to the control terminal. Upon application of the control signal,a decrease in the voltage between the top and bottom terminals to alevel of slightly above 1V was observed, indicating a transformation(which need not involve a switching event) to a more conductive state.The voltage between the top and bottom terminals remained low for theduration of the control signal. In contrast to the results shown in FIG.4, when the control signal was removed, the voltage between the top andbottom terminals returned to its initial value. This result indicatesthat the control signal is able to transitorily create an ON state orON-type state (e.g. a state having some, but not all of thecharacteristics of an ON state, including, for example, conductivitygreater than that of the resistive state) for the device, but that thestate can be terminated by removing the control signal. This resultdemonstrates the non-latching mode of operation of the instant device.In the non-latching mode, application of a control signal can transformthe chalcogenide material to an ON-like state, but the ON-like state isunstable and destroyed when the control signal is removed. Thisindicates that the ON-like state created in non-latching mode existswith a current below the holding current of the chalcogenide materialbetween the top and bottom terminals. Application of the control signalcreates a conductive state (which likely includes a filamentary solidstate plasma or a precursor thereto) while not producing a current thatexceeds the holding current. The control signal injects some degree ofcurrent, but not enough current for the chalcogenide material tostabilize the device in an ON-like state in the absence of the controlsignal.

EXAMPLE 4

In this example, the results of various I-V measurements of theasymmetric three terminal device described in Example 1 are described.The measurements were completed on several different devices selectedrandomly from the devices formed on a wafer and representative resultsare discussed in this example. The measurements were obtained whileoperating the device in a subthreshold mode. In the subthreshold mode,the current between a pair of terminals (e.g. the top and bottomterminals or the control and bottom terminals) is limited to a magnitudebelow the holding current and/or threshold current. As used herein, asubthreshold current refers to a current that is below the holding orthreshold current of the device. (See FIG. 1, the quantum regime, andthe discussion thereof hereinabove.) When the applied current is belowthe holding current, the highly conductive solid state filamentaryplasma is not stable and the device reverts back to its resistive or OFFstate when the signal from the control terminal is removed. Measurementsperformed on the device described in Example 1 indicate that the holdingcurrent of the device is approximately 0.35 mA. In order to operate wellbelow the holding current, a subthreshold current of approximately 1 μAwas supplied between the top and bottom terminals during the course ofthe experiments described in this Example.

FIG. 6 summarizes the results of selected I-V measurements completed bythe instant inventor while operating the device in subthreshold mode.FIG. 6 shows the first quadrant of an I-V plot for the representativethree terminal device described in Example 1. The current I correspondsto the current passing between the load (top) and reference (bottom)terminals of the structure and the voltage V depicted on the horizontalaxis corresponds to the voltage applied between the top and bottomterminals. The I-V relationship between the top and bottom terminals wasdetermined as a function of the control voltage applied to the controlterminal. In the tests, a control voltage of constant magnitude wasapplied to the control terminal and the current between the load andreference terminals was measured as a function of the voltage appliedbetween the top and bottom terminals. The control voltage was applied inthe form of a long duration voltage pulse (e.g. 3 microseconds) and thevoltage between the top and bottom terminals was applied in the form ofa short duration pulse (e.g. 100 nanoseconds) while the control voltagewas being applied. In this example, the control voltage is appliedbetween the control terminal and reference terminal of the device.

The data in FIG. 6 indicate that application of a control voltage to thecontrol terminal may be used to modulate the threshold voltage betweenthe top and bottom terminals. The different I-V curves correspond totests using different control voltages. Separate I-V curves wereobtained for a control voltage of 0V (which was used to determine theholding current and holding voltage of the device) as well as for aseries of control voltages, incremented in steps of 0.25V, between 1Vand 5V. Representative results are shown in FIG. 6. The I-V curvesillustrate the switching transition of the device and show that thethreshold voltage that needs to be applied between the top and bottomterminals varies as the voltage applied to the control terminal varies.The data indicate that the threshold voltage can be varied from a valueof above 4.5V to a value of below 1V by varying the control voltage. Thetest data presented in FIG. 6 demonstrate an ability to modulate thethreshold voltage between two electrodes of a multi-terminal device byapplying a control voltage to a control terminal. In addition tomodulating the threshold voltage between two terminals, the instantmulti-terminal devices may be used to modulate the conductivity of thechalcogenide material between two terminals through application of acontrol signal to the control terminal. The modulation of the currentand threshold voltage between the top and bottom terminals are effectsthat illustrate functionality achievable in the instant multi-terminaldevices that is not available in standard two-terminal devices.

FIG. 7 summarizes the results of the I-V experiments described in FIG.6. FIG. 7 presents the threshold voltage and holding voltage between thetop and bottom terminals as a function of the voltage applied to thecontrol terminal. The threshold voltage is depicted with diamondsymbols, designated as Vth and displayed as the upper curve in FIG. 7.The holding voltage is depicted with square symbols, designated as Vh,and displayed as the lower curve in FIG. 7. The signal applied to thecontrol terminal was a voltage and is designated as V3t in FIG. 7. Theresults indicate that while the holding voltage of the device remainedapproximately constant over the range of control voltages investigated,the threshold voltage exhibited a significant dependence on the controlvoltage. The threshold voltage remained at about 4.5V until the controlvoltage exceeded about 1.5V. Between control voltage of about 1.5V andabout 2V, the threshold voltage exhibited a steep decrease. Thethreshold voltage continued to decrease for control voltages above 2V.

A noteworthy feature of the result depicted in FIG. 7 is thereversibility of the variation in the threshold voltage (and theaccompanying variations in the conductivity and current) of thechalcogenide material of the device. When the signal applied to thecontrol terminal is increased from a low value to a high value, thethreshold voltage decreases and when the signal applied to the controlterminal is decreased from a high value to a low value, the thresholdvoltage increases. The increase and decrease in threshold voltage can beeffected in real time and require no relaxation or other adjustment ofthe device configuration. When an oscillatory or other time-varyingsignal (e.g. AC, sinusoidal, modulated, chopped, intermittent, pulsed)is applied to the control terminal, an oscillatory or time-varyingvariation in the threshold voltage (and/or conductivity and/or currentand/or current density and/or gain) of the chalcogenide material isproduced. The reversibility of operation may also be referred to hereinas bidirectional operation. Reversibility permits one to operate thedevice in the forward (left-to-right) and backward (right-to-left)directions along the V3t axis of FIG. 7. One can establish a particularoperating point along Vth as a function of V3t curve of FIG. 7 andproceed either in the forward or backward directions to establishanother operating point along the curve. Suitable variations in thesignal applied to the control terminal permit one to transform thedevice from one operating state to another operating state on the Vthversus V3t curve.

In one embodiment, reversibility is a feature of the subthreshold modeof operation and/or the non-latching nature of the device while operatedin the subthreshold mode. When a similar experiment is completed on amultiple terminal device that latches, the latching creates an ON stateat or above the holding current that precludes operation of the devicein the subthreshold mode. As a result, a latching device must be relaxedbefore a variation of the control signal manifests an effect on theoperational characteristics of the device. Mere removal of the controlsignal from the control terminal does not suffice to relax the ON stateof a latching device. While operation in latching mode can produce acurve similar to that shown in FIG. 7, the response is unidirectional. Asignal applied to the control terminal can affect the threshold voltagebetween the load and reference terminals, but once the device latches,it remains in the ON state and is insensitive to subsequent variationsin the signal applied to the control terminal until the chalcogenidematerial is allowed to relax back to the OFF state. Once the materialrelaxes, it becomes possible to establish a different threshold voltageby applying a different voltage to the control terminal. The symmetricdevice described in EXAMPLE 2 hereinabove exemplifies a device thatlatches and possesses the unidirectional characteristic describedherein, as confirmed by experiments completed by the instant inventor.

EXAMPLE 5

FIG. 8 shows the I-V response between the top and control terminals ofthe device during operation in subthreshold mode by supplying a currentof ˜1 μA between the top and bottom terminals. The I-V response wasmeasured by varying the voltage between the control terminal and the topterminal and measuring the current passing between the control terminaland the top terminal. FIG. 8 shows the relationship between the currentmeasured between the control terminal and top terminal as a function ofthe voltage applied between the control terminal and top terminal. FIG.8 indicates that a switching event occurred between the top and controlterminals when the voltage applied therebetween reached about 3.5V. I-Vmeasurements were also performed on the symmetric device described inEXAMPLE 2 hereinabove. I-V measurements between the top and controlterminals of the symmetric device demonstrated that a switching eventoccurred in the symmetric device.

The I-V response was also measured between the control terminal andbottom terminal of both the symmetric device described in EXAMPLE 2 andthe asymmetric device described in EXAMPLE 1. FIG. 9 shows therelationship between the current measured between the control terminaland bottom terminal as a function of the voltage applied between thecontrol terminal and bottom terminal for the symmetric device. The I-Vplot of the symmetric device shows a switching effect between controlterminal and bottom terminal.

FIG. 10 shows the relationship between the current measured between thecontrol terminal and bottom terminal as a function of the voltageapplied between the control terminal and bottom terminal of theasymmetric device described in EXAMPLE 1. In contrast to the behaviorobserved for the control terminal and top terminal of the asymmetricdevice and the behavior observed between the control terminal and bottomterminal of the symmetric device, the I-V response measured between thecontrol terminal and the bottom terminal of the asymmetric device showedno indication of a switching effect. Instead, a monotonic increase incurrent was observed as the voltage was increased (including at voltagesabove the voltages for which switching was observed between the top andbottom terminals or the top and control terminals of the asymmetricdevice). The device of this example is thus an embodiment of a devicehaving at least three terminals for which switching can be inducedbetween one pair of terminals (e.g. top and bottom terminals or top andcontrol terminals), but not between another pair of terminals (e.g.control and bottom terminals). The device of this example furtherillustrates an embodiment in which the current passing between twoterminals is less than the holding current of the device when thevoltage applied between the two terminals is greater than or equal tothe threshold voltage.

The device of this example is also an embodiment of a device thatpermits simultaneous application of a signal (e.g. voltage or current)between a first pair of terminals (e.g. top and bottom terminals) and asignal between a second pair of terminals (e.g. control terminal andbottom terminal) without inducing latching or switching of thechalcogenide device. Embodiments of the device permit the simultaneoustransmission of current between two or more pairs of terminals of achalcogenide device having three or more terminals without inducing atransformation of the chalcogenide material from a resistive state to aconductive state. In other embodiments, the instant device permitssequential or pulsed transmission of current between two or more pairsof terminals of a chalcogenide device having three or more terminalswithout inducing a transformation of the chalcogenide material from aresistive state to a conductive state.

EXAMPLE 6

In this example, gain functionality of the device described in EXAMPLE 1is described and demonstrated. A noteworthy aspect of the I-Vcharacteristics depicted in FIG. 10 is the availability of a continuousrange of currents between the control terminal and lower or bottomterminal. The currents available from the third terminal representcurrents that may be injected into the device and the continuous rangeof currents depicted in FIG. 10 distinguishes the instant devices fromconventional devices that show switching between the control terminaland bottom terminal because devices that exhibit switching necessarilyexhibit a discontinuity in the range of currents available from thethird terminal. In FIG. 9, for example, a discontinuous change along thecurrent axis is readily apparent and there is a significant range ofcurrents (from slightly above 0 A to the holding current (which is at orbelow about 0.8 A) that cannot be stabilized between the controlterminal and bottom terminal because the voltages necessary to maintaincurrents in the range induce a switching of the material and areadjustment of the current and voltage to a point along the conductivebranch. In FIG. 10, this limitation in current is not present andcurrents within the range of currents in the unavailable discontinuousrange of FIG. 9 are permitted and may be used to operate the device.This range of currents corresponds generally to the quantum regimedepicted in FIG. 1 and represents currents that extend below the holdingcurrent. In one embodiment, operation within the quantum regime isreversible and the device can be reversibly adjusted in current withinthe range of currents represented by the quantum regime. Currents belowthe holding current or the threshold current can be stabilized andreversibly increased or decreased through adjustment of the signalapplied to the control terminal.

In this example, we demonstrate one example of the new functionalityobtained from operation in the quantum regime and specifically exploitcurrents from within this range to demonstrate gain in the instantdevices. In this example, a voltage of 1V was applied and maintainedbetween the top and bottom terminals of the device described in EXAMPLE1 and a control signal was applied between the control terminal and thebottom terminal. Control signals of various magnitudes were appliedbetween the control terminal and bottom terminal to establish a range ofcurrents that would be unavailable if the chalcogenide material switchedbetween the control terminal and bottom terminal. The variation of thecurrent between the top and bottom terminals as a function of thecurrent supplied between the control terminal and bottom terminal wasmeasured. The results are summarized in FIG. 11, which includes data fortwo regimes of operation. The filled points represent data acquiredwhile the device was operated in the subthreshold mode in which acurrent below the holding current was applied between the controlterminal and the bottom terminal. These points also include a fit to thedata that demonstrates an exponential dependence of the current passingbetween the top and bottom terminals on the current passing between thecontrol and bottom terminals. The superlinear dependence indicates thatthe device provides gain. We note that in a conventional transistor, asuperlinear increase in the current passing between the emitter andcollection terminals is observed as the signal between the base andemitter terminals is increased. An extrapolation of the exponentialdependence indicates that if the device is operated at higher currentlevels in the subthreshold regime, the device may be operated in a modein which variation of a small current between the control terminal andthe bottom terminal permits control of a large current between the topterminal and the bottom terminal. This operational capability is also afeature associated with gain. FIG. 11 also includes a set of data pointsdepicted with unfilled symbols, which corresponds to operation of thedevice in the regime in which the signal supplied to the controlterminal was sufficient to induce a switching transition between the topand bottom terminals. Thus, depending on the signal applied to thecontrol terminal, the device can operate in a pre- or sub-threshold modeor a post-threshold mode.

This example illustrates a device having three or more terminals inelectrical communication with a chalcogenide material in which thedevice exhibits gain.

EXAMPLE 7

In this example, we demonstrate operation in the quantum regime andspecifically utilize currents from within this range to demonstrate gainin the instant devices. In this example, the current-voltage response ofthe device between the top and bottom terminals of the device describedin EXAMPLE 1 is presented for various signals applied to the controlterminal. In the experiments, the current-voltage relationship betweenthe top and bottom terminals was measured when a control signal of aparticular magnitude was applied between the control terminal and thebottom terminal. A series of current-voltage measurements was completedin which signals of different magnitude were applied between the controlterminal and the bottom terminal. Control signals of various magnitudeswere applied between the control terminal and bottom terminal toestablish a different current between the control terminal and bottomterminal, where the range of currents obtained is within the quantumregime and below the holding current. The range of currents would beunavailable if the chalcogenide material switched between the controlterminal and bottom terminal.

The variation of the current between the top and bottom terminals as afunction of the voltage applied between the top and bottom terminals wasmeasured for different levels of current supplied between the controlterminal and bottom terminal. The results are summarized in FIG. 12,which shows a series of current-voltage curves, each of which is labeledaccording to the magnitude of the current passing between the controlterminal and bottom terminal. The results show a dramatic increase inthe slope of the current-voltage curve as the current passing betweenthe control terminal and bottom terminal was increased. The resultsdemonstrate that a small variation in the magnitude of the currentpassing between the control terminal and the bottom terminal can be usedto induce or control a much larger variation in the magnitude of thecurrent passing between the top terminal and the bottom terminal. As anexample of this feature of this embodiment of the instant invention,consider the change in current between the top and bottom terminals thatoccurs when the voltage between the top and bottom terminals is 0.6V. Atthis voltage, when a current of 1.96 mA is passed between the controlterminal and bottom terminal, the current that passes between the topterminal and bottom terminal is approximately 0.003 mA. When the currentbetween the control terminal and bottom terminal is increased to 3.1 mA,however, the current that passes between the top terminal and bottomterminal is approximately 0.35 mA. An increase in the ratio of thecurrent passed between the control terminal and bottom terminal ofslightly more than 1.5 (3.1/1.96) leads to an increase of a factor of100 or more in the current passing between the top terminal and bottomterminal.

This example illustrates a device having three or more terminals inelectrical communication with a chalcogenide material in which thedevice exhibits current amplification.

Corresponding embodiments that include more than three terminals arealso within the scope of the instant invention. In these embodiments,any terminal may function as a control terminal with respect to any twonon-control terminals. Consider as an example a four-terminalchalcogenide device where the terminals are labelled 1, 2, 3, and 4.Terminal 1 may function as a control terminal for terminals 2 and 3, 2and 4 or 3 and 4. Factors such as the relative proximity of terminal 1to terminals 2, 3, and 4 and the voltages present at terminals 2, 3, and4 influence the pair of terminals between which terminal 1 modulates thethreshold voltage or conductivity of the chalcogenide material. If, forexample, a voltage near the threshold voltage is present betweenterminals 2 and 3, while no voltage is present between terminals 2 and4, a control signal provided by terminal 1 is more likely to modulatethe threshold voltage or conductivity between terminals 2 and 3 ratherthan between terminals 2 and 4. Under suitable conditions, it may alsobe possible for terminal 1 to modulate the threshold voltage orconductivity of chalcogenide material between more than one pair ofterminals within the group of terminals 2, 3, and 4. It may also bepossible to provide control signals to two terminals. Control signals toterminals 1 and 2, for example, may modulate the threshold voltage,current, current density, gain or conductivity of chalcogenide materialbetween terminals 3 and 4. Similarly, the relative spatial positioningof the terminals can provide the characteristics associated withsubthreshold and/or non-latching operation as described hereinabove forany subset of three terminals. Analogous arguments hold for embodimentshaving more than four terminals.

Multi-terminal embodiments of the instant devices include devices havingmore than one control terminal and operating under the influence of morethan one control signal. Multiple control signals may be electricalsignals, optical signals or a combination of electrical and opticalsignals.

The terminals of the instant devices may be located in various spatialconfigurations. All terminals, for example, may be in a common plane orlayer or two-dimensional circuit. Alternatively, one or more terminalsmay be positioned outside of a plane in which other terminals reside. Athree terminal device according to the instant invention, for example,may have two terminals and a chalcogenide material in a commonhorizontal layer and a third terminal vertically disposed relative tothat layer. Such a configuration provides for a vertical interconnectioncapability. Analogous embodiments for devices having more than threeterminals are also within the scope of the instant invention.

The instant devices may be combined with other devices or elements toform circuits or networks. In one embodiment, the instant devices may beused as interconnection devices between two or more elements. In thisembodiment, the conductivity of the chalcogenide material present in theinstant device influences the electrical communication between two ormore elements connected to the instant device. A schematic depiction ofthis embodiment is presented in FIG. 12 which shows a circuit or networkelement 200 coupled to a circuit or network element 210 through athree-terminal chalcogenide interconnection device 220. Theinterconnection device 220 includes interconnection terminals 230 and240, control terminal 250 in electrical communication with chalcogenidematerial 260. The elements 200 and 210 may be single devices such astransistors, diodes, silicon devices, other chalcogenide devices orcircuits or networks comprising a plurality of devices. One of theelements may also be a ground.

Application of a control signal to the control terminal of theinterconnection device 220 modulates the conductivity of thechalcogenide material between interconnection terminals 230 and 240,thereby providing a means for controlling the extent of electricalcommunication or signal transmission between elements 200 and 210. Whenthe chalcogenide material 260 is in a resistive state, the conductivityof the interconnection device 220 is low and signal transmission fromelement 200 to element 210 (or vice versa) is poor or non-existent. Theelements 200 and 210 are substantially electrically isolated from eachother so that, for example, currents or voltages generated by one of thetwo elements is substantially not sensed by or substantially does notinfluence the behavior of the other of the two elements. When thechalcogenide material 260 is in a conductive state, the conductivity ofthe interconnection device 220 is high and signal transmission fromelement 200 to element 210 (or vice versa) is good. Electrical voltagesor currents produced by one of the two elements are readily communicatedto the other of the two elements.

As described hereinabove, the state of conductivity of the chalcogenidematerial 260 may be. influenced by applying a suitable control signal tothe control terminal 250. A control signal may induce a transformationof the chalcogenide material from a resistive state to a conductivestate thereby enabling signal transmission and electrical communicationbetween interconnected elements 200 and 210. The magnitude of thecontrol signal required to induce the switching transformation dependson the voltage difference present between the two interconnectionterminals 230 and 240. The greater the voltage difference is, thesmaller in magnitude is the necessary control signal. Removal of acontrol signal or presence of a control signal of insufficient magnitudemay be unable to induce a switching transformation, thereby producing ormaintaining the chalcogenide material in a resistive state andinhibiting signal transmission or electrical communication betweenelements 200 and 210. Corresponding embodiments having more than threeterminals in which a control terminal modulates the threshold voltage orconductivity of chalcogenide material between one or more pairs ofnon-control terminals are also within the scope of the instantinvention.

In another embodiment, a three-terminal chalcogenide device is used tointerconnect three circuit or network elements as shown in FIG. 13. Inthis embodiment, circuit or network elements 400, 410 and 420 areinterconnected to each other through a three-terminal interconnectiondevice 430 that includes interconnection terminals 440, 450 and 460 inelectrical communication with chalcogenide 470. In this embodiment, anyof the three interconnection terminals may be used as a control terminalfor modulating the threshold voltage or conductivity of the chalcogenidematerial between the other two interconnection terminals. As an example,a signal provided by element 410 through interconnection terminal 440may be used as a control signal with respect to the threshold voltage orconductivity of the chalcogenide material between interconnectionterminals 450 and 460, thereby providing for modulation or control ofsignal transmission or electrical communication between elements 400 and420. Element 400 and interconnection terminal 460 may similarly be usedwith respect to elements 410 and 420. Element 420 and interconnectionterminal 450 may similarly be used with respect to elements 400 and 410.In this embodiment, the magnitude of a control signal is determined bythe signal produced by a circuit or network element. Correspondingembodiments in which chalcogenide devices with more than three terminalsare used to interconnect more than three circuit or network elements arealso within the scope of the instant invention.

Although the schematic depictions of FIGS. 12 and 13 indicateinterconnection of circuits or network elements in a two-dimensionalconfiguration, interconnection in three-dimension is also within thescope of the instant invention. One or more terminals may be verticallydisposed or otherwise non-co-planar with the chalcogenide material orother terminals. A control signal, for example, may be provided from aterminal or device orthogonal to a plane in which a chalcogenidematerial may reside.

In other embodiment, the instant multi-terminal devices may providesignals to other devices or elements in circuits or networks. Asdescribed hereinabove, when a sub-threshold voltage is applied betweentwo terminals (e.g. load and reference terminals) of a chalcogenidedevice, it is possible to induce a switching of the chalcogenidematerial between those two terminals through application of a controlsignal provided by a control terminal in electrical communication withthe chalcogenide material. The switching is accompanied by a decrease inthe magnitude of the voltage and an increase in the magnitude of thecurrent between the load and reference terminals. These changes involtage and current may be used as input signals to other devices orelements in a circuit or network. As an example, consider the devicearrangement described in FIG. 12 hereinabove where the voltage acrossinterconnection terminals 230 and 240 is a sub-threshold voltage and thechalcogenide material is in a resistive state. If a control signal ofcritical magnitude is subsequently applied to the control terminal 250,a switching of the chalcogenide material between interconnectionterminals 230 and 240 to a conductive state occurs. The switching isaccompanied by voltage and current changes between interconnection 230and 240, as described hereinabove, and these voltage and current changesmay be provided as input or driving signals to element 210 and/orelement 200. The principles and modes of operation described herein forthree-terminal embodiments of the instant invention extend analogouslyto multi-terminal devices having more than three terminals.

The disclosure and discussion set forth herein is illustrative and notintended to limit the practice of the instant invention. Numerousequivalents and variations thereof are envisioned to be within the scopeof the instant invention. It is the following claims, including allequivalents, in combination with the foregoing disclosure, which definethe scope of the instant invention.

1. An electronic device comprising: a first terminal; a second terminal;a third terminal; and a material capable of supporting a space-chargeregion; said space-charge material being in electrical communicationwith said first terminal, said second terminal and said third terminal;said space-charge material having a first space-charge region present atsaid first terminal, a second space-charge region present at said secondterminal, and a third space-charge region present at said thirdterminal; each of said space-charge regions comprising an equilibriumdistribution of spatially-separated charged species, said chargedspecies including positive and negative charges; wherein application ofan electrical signal to said third terminal modifies said equilibriumdistribution of said charged species in said first space charge region.2. The electronic device of claim 1, wherein application of saidelectrical signal to said third terminal further modifies saidequilibrium distribution of charged species in said second space chargeregion.
 3. The electronic device of claim 1, wherein application of saidelectrical signal to said third terminal injects electrons into saidfirst space charge region.
 4. The electronic device of claim 1, whereinapplication of said electrical signal to said third terminal injectsholes into said first space charge region.
 5. The electronic device ofclaim 1, wherein application of said electrical signal to said thirdterminal ionizes a defect of said first space charge region.
 6. Theelectronic device of claim 1, wherein application of said electricalsignal to said third terminal forms an electric field in the vicinity ofsaid third terminal, said electric field causing said modification ofsaid equilibrium distribution of said charged species in said firstspace charge region.
 7. The electronic device of claim 1, wherein saidmodification of said equilibrium distribution of charged species in saidfirst space-charge region induces an interaction between said firstspace-charge region and said second space-charge region.
 8. Theelectronic device of claim 7, wherein said interaction between saidfirst space-charge region and said second space-charge region is aquantum interaction.
 9. The electronic device of claim 8, wherein saidquantum interaction includes tunneling.
 10. The electronic device ofclaim 1, wherein said material capable of supporting a space-chargeregion is a chalcogenide material.
 11. The electronic device of claim 1,wherein said material capable of supporting a space-charge region is aninsulating material.
 12. The electronic device of claim 1, wherein saidmaterial capable of supporting a space-charge region is a semiconductingmaterial.